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Feature

Ice core from Antarctica with an aggregation of soil grains. These dark soil grains absorb more sunlight than the surrounding ice and become warmer. This heating causes small amounts of melting of the surrounding ice and allows the grains to migrate through the ice to form this aggregate. Credit: Hans Paerl, University of North Carolina at Chapel Hill

Image of the Meridiani Planum sediments inside Endurance crater taken by Microscopic imager on sol 145. This image was taken after grinding with the Rock Abrasion Tool (RAT) and reveals micro-scale textures within the rock. No obvious crystals or grains are visible indicating that this outcrop is made up of aggregates of much smaller particles. Significant porosity is also revealed here presumably from recrystallization and dissolution processes that occurred after the sediments were deposited. The image is about 30 millimeters across. Credit: NASA/JPL/Cornell/USGS

Researchers at NASA's Johnson Space Center in Houston and Université Paris Sud have found that the rock deposits at ‘Meridiani Planum’ on Mars could be remnants of a massive, ancient ice deposit.

This new interpretation of the data returned by the Mars rover Opportunity does not require warmer climates on Mars as has been previously proposed by members of the Mars Exploration Rover science team. Instead, the researchers conclude that many of the chemical and mineralogic features observed in these rocks can best be explained by interactions between dust and small films of liquid water inside a massive ice deposit under cold, dry climactic conditions.

In a paper posted as an “Advance Online Publication” for the March issue of the journal Nature Geoscience, Paul Niles, NASA space scientist at JSC, and Joseph Michalski, researcher at the Institut d’Astrophysique Spatiale, Université Paris Sud, Orsay, France, propose that acidic conditions are created by a mechanism very similar to acid rain on Earth when atmospheric sulfur dioxide aerosols are incorporated into snowfall and subsequently into the ice deposit. These aerosols react with thin films of water and mineral aggregates inside small pockets within the ice when they are exposed to radiant heating from the sun.

These conditions would have been ideal for forming sulfate minerals. The ice would have served as an efficient pathway for collecting and concentrating the sulfur dioxide aerosols and dust floating around in the atmosphere of ancient Mars.

"This is an exciting idea because it suggests that many of the sulfate minerals found all across the surface of Mars formed from weathering in massive ice deposits rather than from processes involving large amounts of liquid water," said Niles. "Since these sulfate minerals are frequently associated with valleys and channels, this new idea provides a new way to interpret many of the features that have been used to argue for lakes, rivers and oceans on Mars. Thus, we are proposing that even ancient Mars had a climate similar to the one we see today."

The region of Meridiani Planum is near the equator and cannot currently sustain large ice deposits. The authors propose that the ice could have formed in ancient times, when the poles were in a different place or when the Martian axis of rotation was tilted at a different angle to its present-day orbit around the sun.

As the climate near Meridiani changed, these massive ice deposits would have sublimed away, leaving behind a residue of fine-grained aggregates of sulphate-rich material. This material was reworked by the winds of Mars and deposited in the layers observed by the Opportunity rover.

The results of this study imply a common formation mechanism for nearly all the sulfate minerals formed on Mars. In particular this process provides a plausible explanation for the formation of similar deposits of hematite and sulfate minerals found scattered across nearly half of the equatorial region of Mars, including deposits in Aram chaos and Valles Marineris. This could also be a plausible explanation for the sulfates and layered deposits targeted in the proposed Mars Science Laboratory landing sites.